What’s the cheapest way to travel from London to Amsterdam?

My parents are going to London in a few weeks and my mom emailed me to find out what is the cheapest way to travel from London to Amsterdam.  I’ve never been to either of those places but I always wanted to go.  So I hit the internet and started my search.  My parents will be traveling from London to Amsterdam from Wednesday to Friday.  Here is what I found:

1. Easy Jet - 88 £ each return.

2. Overnight Ferry from London to Amsterdam - I have heard that taking a ferry from London to Amsterdam is cheap, however this isn’t an option since my parents don’t have a lot of time.  Some of the ferry operators include -  DFD Seaway, Stena Line, PO Ferries .  I did do a search on DFD Seaway just to see how much it would cost and it came to 80 £ each return.  This price doesn’t include the cost of the cabin. You have to purchase that separately.

3. British Airways – 106 £ each return

4. KLM - 166 £ each return

5. CityJet - This was the most expensive – 300 £ each return

So it looks like Easy Jet is the winner! Have you ever traveled from London to Amsterdam? What was the cheapest way for you?